There's really no surface where Andy is the best in the World so he needs to exploit the bad moments of Djokovic, Nadal, and Federer. And he's at around the same age as Rafa and Nole so he can't wait until they are declining, unless they have injuries like Rafa has recently had. I think Djokovic and maybe Rafa are just too good for him on hardcourts. On grass he's closer to the best, yet I could see players like Tsonga, Berdych, and Raonic potentially threatening him on fast courts. So, I could see him winning yet 1-3 slams; if he can consistently play his best game, there will be HC/grass slams where none of Djokovic, Nadal, and Federer play their best tennis. But eventually the new generation will rise so he won't have many years of great opportunites.

I'd say 3 or 4. With Nole still there and Nadal really resurging and with the guys like Wawrinka, Berdych, and Del Po starting to really push for those top spots more than in the past, I think it'll be too difficult for Murray to really start dominating and racking up slams.